This invention relates generally to demodulators for amplitude-shift keyed (ASK) signals and, more particularly, to demodulators that demodulate digital ASK data signals whose data bits are each characterized by either an absence of pulses or a burst of pulses at a predetermined frequency.
Demodulators of this particular kind are commonly used in optical communication systems, as part of a receiver that receives optical signals modulated according to ASK data signals. The data signals carry digital data in the form either of an absence of light pulses or a burst of light pulses, occurring at a predetermined frequency. In one common system, the data rate is 75 kilobits per second, and the light pulse bursts each have a duration of 6.67 microseconds, at a frequency of 1.5 MHz. Each burst thus incorporates ten light pulses.
One characteristic of optical communication systems of this kind is that the signal strength of the ASK data signals being received can vary over a range as high as 5000 to 1. This can pose a significant design problem when a signal of relatively high strength is followed immediately by a signal of relatively low strength. The receiver""s latency period is a measure of how quickly it can recover from the receipt of a signal of relatively high strength so as to properly receive a signal of relatively low strength. Latency periods of less than 100 microseconds are desired.
In the past, some demodulators of this kind have achieved low latency periods by feeding back a signal to the receiver from a downstream processor, indicating that a data frame has concluded and that the receiver should reset itself to await the receipt of a new data signal. Such demodulators are generally effective in providing the desired low latency periods; however, there is a need for an ASK demodulator that can provide a similar low latency period without the need to feed back a reset signal from a downstream processor. The present invention fulfills this need and provides further related advantages.
The present invention is embodied in an improved demodulator, and related method, for demodulating amplitude-shift keyed (ASK) data signals, in which data bits are each characterized by either an absence of pulses or a burst of pulses at a predetermined frequency, wherein the signal strength of the data signal can vary by several orders of magnitude. The demodulator includes a bandpass amplifier that receives and amplifies the ASK data signal, the amplifier being tuned to the predetermined frequency of the bursts of pulses, and it further includes an edge detector that detects the envelope of the amplified signal, to produce an envelope signal. A comparator compares the envelope signal with a prescribed threshold, to produce a pulse signal indicative of the presence of a burst of pulses in the ASK data signal. Finally, a pulse stretcher increases the length of each pulse in the pulse signal to a length substantially the same as the expected length of the corresponding burst of pulses in the ASK data signal.
In other, more detailed features of the invention, the ASK data signal is an optical signal, and the apparatus further incorporates a device, e.g., a photodiode, for converting that optical signal to a corresponding electrical signal. In addition, the bandpass amplifier has a substantially constant gain, which leads to the envelope signal having a duration that varies according to the strength of the original ASK data signal. The pulse stretcher can take the form of a mono-stable multivibrator, for causing the successive pulses in the pulse signal to have durations substantially the same as the expected nominal durations of the pulse bursts in the original ASK data signal.
Other features and advantages of the present invention should be apparent from the following description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.